Optical sampling of voltage and charge at the internal nodes of analog and digital integrated circuits has been accomplished in both Gallium Arsenide and Silicon. Though the physical mechanisms of the sampling are different for the two material systems, both techniques require low noise, narrowband receiver electronics. Another limitation of sampling technology as it is now practiced is that the signal repetition rate must be at a harmonic of the sampling rate. In the case of optical sampling techniques, the signal repetition rate is governed by the laser cavity itself. In order to take advantage of the higher bandwidths possible with faster lasers. This invention provides a technique that allows the sampling rate to exceed the signal repetition rate, or fundamental. In accordance with this invention, we provide a sampling receiver design which is generally useful for any noisy sampling operation. With this sampling receiver, which we call the Golay Sampling Receiver, it is possible to use very high sampling rates together with narrow banding to recover very small signals in the presence of noise.
Background for this invention is discussed in several papers on electro-optic sampling. For example, see K. J. Weingarten, et al, "Picosecond Optical Scanning of GaAs Integrated Circuits", IEEE Journal of Quantum Electronics, JQE-24, No. 2, February 1988; B. H. Kolner et al, "Electro-optic Sampling With Picosecond Resolution", Electronics Letters, Vol. 19, pp. 574-575; H. K. Heinrich, et al, "Noninvasive Sheet Charge Density Probe Of Integrated Silicon Devices" Applied Physics Letters, Vol. 48, pp. 1066-1068, 1986; and H. K. Heinrich et al, Measurement of Real Electronics Letters, Vol. 19, pp. 574-575; H. K. Heinrich, et al, "Noninvasive Sheet Charge Density Probe Of Integrated Silicon Devices" Applied Physics Letters, Vol. 48, pp. 1066-1068, 1986; and H. K. Heinrich et al, "Measurement of Real time digital Signals In A Silicon Bipolar Junction Transistor Using A Noninvasive Optical Probe", Electronic Letters, Vol. 22, pp. 650-652, 1986.
The following prior art patents relate to electro-optic sampling of electronic circuits:
U.S. Pat. No. 4,875,006, Henley et al; PA1 U.S. Pat. No. 4,745,361, Nees et al; PA1 U.S. Pat. No. 4,681,449, Bloom et al; and PA1 U.S. Pat. No. 4,446,425, Valdmanis et al.
Henley et al describes a high speed test system for semiconductor integrated circuits. The system comprises an adapter board for receiving a circuit for test, a plurality of driver circuits positioned around the adapter board for applying test patterns and voltages to the integrated circuit, and an electro-optic sensor. The driver circuits are connected to contacts of the circuit undergoing test, which are in turn connected to the electro-optic sensor. A laser provides light sampling pulses through the electro-optic sensor, and directs reflections of the sampling pulses from the sensor to an electro-optic voltage measuring means. Finally, a central means is provided for controlling the system.
Ness et al relates to an electro-optic network analysis system, which uses electro-optic sampling. The device under test is integrated on a substrate of electro-optic semiconductor material, and is connected to transmission lines on the substrate. Electro-optically generated sampling signals propagate along the transmission lines toward and away from the device under test. The signals are electrically sampled by a laser pulse sampling beam, which is responsive to the change in refractive index due to the signal at locations equidistant from the generation position. The waveform resulting from electro-optic sampling near the device under test corresponds to the sum of the signal incident upon the device and the signal reflected therefrom, while the waveform resulting from electro-optic sampling at the location away from the device corresponds to the incident waveform. The waveforms are mathematically processed and Fourier transformed to derive the scattering parameters.
Bloom et al describes a non-contact test system for high speed testing of electronic circuits by electro-optic sampling. A laser optically generates signals in a compound semiconductor. The signal is transmitted to a microstrip on the semiconductor surface. A second polarized laser signal is passed through the crystal, and its polarization is modulated by the electric field in the microstrip. The polarization presents a measure of the field strength and the signal. An equivalent time representation of the sampled signal can be obtained by varying the relative delay between two beams.
Valdmanis et al relates to a system for the measurement of electrical signals by electro-optic sampling of the signal being analyzed in a traveling wave Pockels cell. Laser sampling pulses of subpicosecond duration are transmitted through the cell as polarized light, and translated into a difference output corresponding to the difference in amplitude between the transmitted and rejected components of the polarized light. Signals, which are synchronous with the optical sampling pulses, are generated to propagate along the cell in a direction which is transverse to and in variably delayed relationship with the transmission of the optical sampling pulses. A separate beam of the optical pulses is desirably chopped and used to activate a photoconductive device which produces the signals. The difference output is processed and displayed on a time basis which is synchronous with the variable delay of the pulses. The signal is displayed on an expanded time scale for measurement and other analysis.